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Electron-withdrawing groups exert an "inductive" or "electron-pulling" effect on covalent bonds. The strength of the electron-withdrawing group is inversely proportional to the pKa of the carboxylic acid. [2] The inductive effect is cumulative: trichloroacetic acid is 1000x stronger than chloroacetic acid.
An electron withdrawing group (EWG) will have the opposite effect on the nucleophilicity of the ring. The EWG removes electron density from a π system, making it less reactive in this type of reaction, [ 2 ] [ 3 ] and therefore called deactivating groups .
The +M effect, also known as the positive mesomeric effect, occurs when the substituent is an electron donating group. The group must have one of two things: a lone pair of electrons, or a negative charge. In the +M effect, the pi electrons are transferred from the group towards the conjugate system, increasing the density of the system.
The substituent R next the amine methylene bridge is an electron-withdrawing group. The original 1928 publication by Thomas S. Stevens [2] concerned the reaction of 1-phenyl-2-(N,N-dimethylamino)ethanone with benzyl bromide to the ammonium salt followed by the rearrangement reaction with sodium hydroxide in water to the rearranged amine.
The captodative effect is the stabilization of radicals by a synergistic effect of an electron-withdrawing substituent and an electron-donating substituent. [2] [3] The name originates as the electron-withdrawing group (EWG) is sometimes called the "captor" group, whilst the electron-donating group (EDG) is the "dative" substituent. [3]
In the context of the Diels–Alder reaction, the transition state in which the most significant substituent (an electron-withdrawing and/or conjugating group) on the dienophile is oriented towards the diene π system and slips under it as the reaction takes place is known as the endo transition state.
The Cr(CO) 5 moiety is a strong electron withdrawing group making the α-proton highly acidic. A methoxy chromium carbene with a methyl side chain has a pK a of 12.5 in aqueous acetonitrile (1:1 volume ratio). [8] [9] For comparison, methyl acetate has a pK a of 25.6, demonstrating the strong electron withdrawing nature of the Cr(CO) 5 moiety.
[3] [10] Many electron-withdrawing groups have been shown promote the cyclopropanation including ketones, esters, amides (the example below involves a Weinreb amide), sulfones, nitro groups, phosphonates, isocyanides and even some electron deficient heterocycles. [15] Cyclopropanation with the Johnson–Corey–Chaykovsky reaction